1. Field of the Invention
The invention generally relates to attenuator circuits for reducing background noise signals, and more particularly to an artifact signal attenuator circuit for a heart-rate sensor of the type most specifically defined and claimed in U.S. Pat. No. 3,769,974 which issued Nov. 6, 1973.
2. Description of the Prior Art:
As disclosed in the aforementioned United States Letters Patent, a pulse transducer contained in a package of a wristwatch-size including a plurality of light emitting diodes which serve as a source of light may be employed to provide an output signal which varies as a function of the pulse rate of a living body. The emitted light is transmitted through the skin of the body to blood vessels from which it is reflected to a sensor with an intensity varying as a function of variations in the pulse wave pressure of the artery. Unfortunately, it has been found that light also is reflected from the skin of the body. This light also is detected by the sensor, whereupon a dc signal, hereinafter referred to as an artifact signal, appears at the output of the transducer.
In practice, the artifact signal consists of two parts; namely, a constant level dc signal, resulting from a constant reflection from the skin, and, a sinewave signal varying as reflection is varied in response to motion imparted to the sensor. The constant signal component achieves a dc level typically one hundred to two hundred times greater than the amplitude of the heart signal present at the sensor's output. When the sensor is moving with respect to the skin, a sinewave component appears around the dc level of the aforementioned artifact signal component. As should be apparent, the components of the artifact signal must be removed if the relatively small heart signal, typically ten to twenty millivolts, is to undergo high amplification.
In the presence of one-to-two volt signal components for the artifact signal the gain required to increase the amplitude of a ten-to-twenty millivolt heart signal to a useable level tends to saturate available amplifiers. The most straight-forward way to eliminate the components of the artifact signal would be to effect an ac-couple of the sensor output to filter/amplifier circuits. Unfortunately, for the signal spectrum of interest, 0.5 Hz to 4 Hz, the coupling capacitor simply is too large to meet the packaging requirements necessary to provide the electronics in a wristwatch-size package.
It is, therefore, the general purpose of the instant invention to provide a suitable attenuator circuit adapted to be connected with the output of a pulse transducer and the input of subsequent signal processing circuits, mounted in a package of wristwatch-size, for purposes of reducing the amplitude of components of an artifact signal at the sensor's output so that the heart signal provided thereby can be filtered and amplified by the further signal processing circuits.